1. Technical Field
The present invention relates to a light-emitting device comprising a light source which emits light having a plurality of colors, a display apparatus using the light-emitting device, and a read apparatus using the light-emitting device.
2. Description of the Related Art
It has been conventionally known that, in some types of transmissive liquid crystal which employ a backlight including a side light, and reflective liquid crystals which employ a front light, a light-emitting device, which includes a white cold cathode fluorescent tube or a white light-emitting diode (LED) as a light source, is mounted as a back light or a front light for display. Particularly, many types of cellular phones which have rapidly become popular recently-employ a white LED.
However, a light source using a white cold cathode fluorescent tube and a white LED have a problem that white point and luminance characteristics vary largely depending on changes in temperature characteristics and changes over time. In order to solve this problem, the following two methods have been proposed, for example.
The first method is effective in the case where multiple types of light sources emitting light of different colors are switched by a time-division to provide a white light source. As described in Japanese Laid-Open Publication No. 10-49074, for example, light sources of respective colors are monitored by an optical sensor and changes in amounts of light are fed back to respective light sources for emitting white light.
The second method is effective for the case where multiple types of light sources emitting light of different colors are made to emit light at the same time to provide a white light source. As described in Japanese Laid-Open Publication No. 11-295689, light sources of respective colors are monitored by an optical sensor and changes in amounts of light are fed back to respective light sources so as to have an equal value as a certain predetermined value for emitting white light.
General examples of light-emitting operations of light sources for allowing the multiple types of light sources to emit light at the same time and the colors of emitted light to be mixed for providing white color in the second method mentioned above are shown in FIGS. 12 and 13. The multiple types of the light sources are, for example, a red LED, a green LED, and a blue LED Methods for controlling a light-emitting operation of the light sources are roughly divided into two types: a pulse width control method shown in FIG. 12; and a current value control method shown in FIG. 13. A method which combines these two methods is also possible.
FIGS. 12(a), (b) and (a) are graphs which respectively show the performance of pulse width control of current values flowing through the red, green and blue light sources, with the horizontal axes indicating time and the vertical axes indicating current value. By performing pulse width control of the emission intensities of the light sources, i.e., by controlling the time lengths of the light emitted by the light sources while the emission intensities of the light sources are maintained constant, apparent light emission intensities change. For example, in order to increase the apparent light emission intensities, the light emitting time of the light sources is lengthened. In order to reduce the apparent emission Intensities, the light emitting time of the light sources is shortened. In this way, the apparent light intensities of the light sources are controlled by adjusting the length of time while light is emitted and the length of time while light is not emitted.
Taking the light-emitting operation of the red light source as shown in FIG. 12(a) as a standard, the green light source as shown in FIG. 12(b) emits light for a period of time shorter than that of the red light source in the first cycle. In the next cycle, the green light source emits the light for a further shorter time to reduce the apparent emission intensities. The blue light source as shown in FIG. 12(c) emits light for a period of time longer than the red light source. In the next cycle, the blue light source emits light for further longer time to increase the apparent emission intensities.
As described above, in the pulse width control method, the light-emitting time of the light sources are controlled at a predetermined frequency while the values of the current flowing through the light sources are maintained constant. The frequency should be set to a cycle which is not perceived by the eyes of a human, for example, 60 Hz or higher. If the frequency is set too high, the cost for the driving circuit increases. Thus, generally the frequency is set to about 200 Hz.
Similarly to FIG. 12. FIGS. 13(a), (b) and (a) are graphs which respectively show sequentially changing current values flowing through the red, green and blue light sources, with the horizontal axes indicating the time and the vertical axes indicating the current values. In this case, by sequentially changing the amount of the current flowing through the light sources over time, the emission intensities of the light sources is controlled. In order to increase the emission intensities, the current value is increased. In order to reduce the emission intensities, the current value is reduced. For example, in the red light source as shown in FIG. 13(a), the emission intensity is increased by increasing the current values flowing through the red light source. In the green light source as shown in FIG. 13(b), the emission intensity is reduced by reducing the current values. As shown in FIG. 13(c), the emission intensity may be maintained constant by allowing a current which is constant in terms of time to flow.
The first and the second methods described above have the following problems. First, the time-division switching method described in Japanese Laid-Open Publication No. 10-49074 has an advantage that the emission intensities of the light sources can be monitored by a single type optical sensor, but the method has a critical problem that it is effective for only the time-division method, in which light sources are turned on one type at a time in turn, and it cannot be applied to a method other than the time-division method.
Further, the simultaneous light-emitting method described in Japanese Laid-Open Publication No. 11-295689 has a problem that the cost is high because a color separation filter is necessary in addition to three types of optical sensor corresponding to the red, green, and blue light sources, and a problem that control of the emission intensities becomes inaccurate due to a variance in optical sensor outputs because three types of optical sensor cannot be located at the same place.
Further, although it is desirable that the backlight emits light uniformly across its entire surface, it is difficult to actually emit light in a uniform manner. Thus, uneven luminance is usually generated. It is also a concern that, when three types of the light sources, i.e., a red light source, a green light source, and a blue light source are used instead of a light source emitting white light, uneven color may be generated because the colors of the light from the light sources are not perfectly mixed. In the case where such uneven luminance or uneven color is generated, variance may be a problem depending on where the display apparatus is located.